1. Field of the Invention
The present invention relates to a space-vector pulse-width modulation method for a use in a frequency converter. In particular, the invention relates to a space-vector pulse-width modulation method for a frequency converter provided with a voltage intermediate circuit. The invention also relates to a voltage-controlled frequency converter controllable by a space-vector pulse-width modulation method.
2. Discussion of the Background
Space-vector pulse-width modulation (Space vector PWM, SVPWM) is a pulse-width modulation method for a frequency converter provided with a voltage intermediate circuit that is particularly well suited for digital implementation. In the modulation method, the on-time is generally calculated beforehand for two output voltage vectors of the frequency converter (i.e. for two software positions in the inverter bridge of the frequency converter) by software, from which the requested output voltage space vector is generated as an average. A control system containing a digital counter then takes care of changing the switch positions so that these two switch positions are on for the calculated times in question. The times are calculated by using the output voltage space vector as an input variable.
In the calculation of the on-times of the switch positions in question, the intermediate circuit voltage is assumed to be constant. This assumption holds good in the case of an intermediate circuit designed to so-called traditional ratings. If for some reason the intermediate circuit voltage undergoes a considerable change during a switching period, then the modulation method will be unable to implement the requested average output voltage space vector.
In the following, prior-art space-vector PWM, which is described e.g. in reference H. van der Broeck, H. Skudelny, and G. Stanke, “Analysis and realization of a pulse width modulator based on voltage space vectors”, in IEEE-IAS Conf. Records, pages 244–251, Denver, USA, 1986, will be referred to as traditional space vector PWM or SVPWM for short. The input variable in SVPWM is an output voltage reference, which can be divided into real and imaginary componentsUs,ref=Ux+jUy  (1)
The inverter output voltage can be expressed as a space vector
                                          U                          s              ,              out                                ⁡                      (                          sA              ,              sB              ,              sC                        )                          =                              2            3                    ⁢                                    U              DC                        ⁡                          (                              sA                +                                  sB                  ·                                      ⅇ                                          j                      ·                      2                      ·                                              π                        /                        3                                                                                            +                                  sC                  ·                                      ⅇ                                          j                      ·                      4                      ·                                              π                        /                        3                                                                                                        )                                                          (        2        )            
where s{A,B,C} is 1 if the phase {A,B,C} is connected to the upper arm of the inverter and 0 if the phase is connected to the lower arm. In different combinations, 6 active voltage vectors and two so-called zero vectors (all phases connected to the same arm) are obtained. The directions of the active voltage vectors in the complex plane are 0, 60, 120, 180, 240 and 300 degrees.
As shown in FIG. 1, the complex plane is divided into six equal sectors, with the first sector starting from the real axis. The direction of the real axis corresponds to the direction of the magnetic axis of the A phase of the stator of a three-phase motor connected to the inverter, direction 120 degrees corresponds to the direction of the magnetic axis of the B phase of the motor and 240 degrees to the direction of the C phase. The active voltage vectors form the borders between these sectors.
It is now possible in any sector m to produce any average voltage vector (of limited magnitude, however) by using the voltage vectors Vm and Vm+1 at the sector borders for time Tm and Tm+1. Space vector PWM implements such a voltage vector when the voltage reference is Us,ref.
                                          U                          s              ,              ref                                =                                                                      T                  m                                                  T                  s                                            ⁢                              V                m                                      +                                                            T                                      m                    +                    1                                                                    T                  s                                            ⁢                              V                                  m                  +                  1                                                                    ,                            (        3        )            
Ts is the sampling period, i.e. the update interval of the voltage reference. The voltage vectors Vm and Vm+1 at the borders of the sector m can be defined by equations
                                          V            m                    =                                    2              3                        ⁢                          U              DC                        ⁢                          ⅇ                              j                ⁢                                  π                  3                                ⁢                                  (                                      m                    -                    1                                    )                                                                    ⁢                                  ⁢                              V                          m              +              1                                =                                    2              3                        ⁢                          U              DC                        ⁢                                          ⅇ                                  j                  ⁢                                      π                    3                                    ⁢                                      (                    m                    )                                                              .                                                          (        4        )            
From equations (3) and (4), it is possible to calculate the times Tm and Tm+1 for the counter
                              [                                                                      T                  m                                                                                                      T                                      m                    +                    1                                                                                ]                =                                                                                                  3                    ⁢                                          T                      s                                                                                        U                  DC                                            ⁡                              [                                                                                                    sin                        ⁡                                                  (                                                                                    π                              3                                                        ⁢                            m                                                    )                                                                                                                                    -                                                  cos                          ⁡                                                      (                                                                                          π                                3                                                            ⁢                              m                                                        )                                                                                                                                                                                                  -                                                  sin                          ⁡                                                      (                                                                                          π                                3                                                            ⁢                                                              (                                                                  m                                  -                                  1                                                                )                                                                                      )                                                                                                                                                              cos                        ⁡                                                  (                                                                                    π                              3                                                        ⁢                                                          (                                                              m                                -                                1                                                            )                                                                                )                                                                                                                    ]                                      ⁡                          [                                                                                          U                      x                                                                                                                                  U                      y                                                                                  ]                                .                                    (        5        )            
For the remaining time of the switching period Ts, the zero vector is usedT0=Ts−Tm−Tm+1 
SVPWM can be implemented as a so-called symmetric method. In that case, the zero vector is used at the beginning, middle and end of the switching period, and the active voltage vectors Vm and Vm+1 are divided into two parts. Symmetric implementation provides the advantage of a lower harmonics content.